1. Field of the Invention
The present invention relates to a laser apparatus in which laser beams emitted from a plurality of semiconductor laser elements are collected by an optical condensing system, and are optically multiplexed in an optical fiber.
2. Description of the Related Art
The following documents (1) to (5) disclose information related to the present invention.
(1) Japanese Unexamined Patent Publication No. 2002-202442 (corresponding to U.S. Patent Laid-Open No. 20020090172)
(2) Japanese Unexamined Patent Publication No. 11(1999)-54852
(3) U.S. Pat. No. 5,392,305
(4) Japanese Unexamined Patent Publication No. 11(1999)-167132 (corresponding to U.S. Pat. No. 6,404,786)
(5) Japanese Unexamined Patent Publication No. 11(1999)-87814
Conventionally, in order to generate a laser beam having an ultraviolet wavelength, wavelength conversion lasers, excimer lasers, and Ar lasers are used. In the wavelength conversion lasers, infrared light emitted from a solid-state laser excited with a semiconductor laser is converted into a third harmonic having an ultraviolet wavelength. Further, GaN-based compound semiconductor lasers which emit a laser beam having a wavelength in the vicinity of 400 nm have been provided.
Light sources which emit laser beams having the wavelengths as mentioned above are being considered to be used in exposure systems for exposure of photosensitive materials which are sensitive to light in a predetermined wavelength range including an ultraviolet wavelength range of 350 to 420 nm. In such a case, the light sources for exposure are required to have sufficient output power for exposing the photosensitive materials. The above predetermined wavelength range is hereinafter referred to as the ultraviolet range.
However, the excimer lasers are large in size, and the manufacturing costs and maintenance costs of the excimer lasers are high.
In the wavelength conversion lasers which convert infrared light into a third harmonic in the ultraviolet range, the wavelength conversion efficiency is very low. Therefore, it is very difficult to achieve high output power. In addition, since an optical wavelength conversion element is used in the above wavelength conversion lasers, and the optical wavelength conversion element is expensive, the manufacturing cost of the wavelength conversion laser is high.
Further, the efficiency in electric-to-optical conversion in the Ar lasers is as low as 0.005%, and the lifetime thereof is as short as about 1,000 hours.
On the other hand, since it is difficult to obtain a low-dislocation GaN crystal substrate, an attempt has been made to achieve high output power and reliability in a GaN-based compound semiconductor laser. In the attempt, a low-dislocation region having a width of about 5 micrometers is produced by a growth method called ELOG (epitaxial lateral overgrowth), and a laser region is formed on the low-dislocation region. However, even in the attempt, it is difficult to obtain a low-dislocation substrate having a large area. Therefore, GaN-based compound semiconductor lasers having a high output power of 500 mW to 1 W are yet to be commercialized.
In consideration of the above circumstances, optically-multiplexing laser-light sources which can increase output power have been proposed, for example, as disclosed in aforementioned document (1). In the optically-multiplexing laser-light sources, laser beams emitted from a plurality of semiconductor laser elements are optically multiplexed in a multimode optical fiber.
However, the above optically-multiplexing laser-light sources have a drawback that light-emission end faces of the semiconductor laser elements and optical elements such as lenses and the optical fiber are contaminated with materials (contaminants) remaining in a sealed container, and laser characteristics deteriorate. Typical examples of the contaminants are hydrocarbon compounds. It is known that laser light polymerizes or decomposes the hydrocarbon compounds, and the polymerized or decomposed products adhere to the light-emission end faces and the optical elements, and prevent the increase in the output power.
In addition, for example, aforementioned document (2) discloses that a photochemical reaction between oxygen and low molecular siloxane suspended in air occurs, and reaction products, SiOx, are deposited on an optical glass window element. Therefore, periodic replacement of window elements exposed to air has been recommended, for example, as indicated in document (2).
In order to solve the above problems, for example, aforementioned document (3) has proposed to mix oxygen of 100 ppm or more into sealing gas so that the hydrocarbon compounds and the like are decomposed.
Further, in optical systems in which ultraviolet light having a wavelength of 400 nm or less is applied to optical elements, arrangement of the optical elements in an atmosphere containing nitrogen of 99.9% or more has been proposed, for example, as indicated in aforementioned document (4).
Furthermore, oil removal from and cleaning of the inside of a laser apparatus has been proposed, for example, as indicated in aforementioned document (5).
Nevertheless, in optical systems having an external resonator and generating ultraviolet light as disclosed in document (4), expensive equipment for supplying highly-pure nitrogen is necessary for purging the external resonator with nitrogen of 99.9% or more. Therefore, an increase in the manufacturing cost of the laser apparatus cannot be avoided.
On the other hand, there has been disclosed in Japanese patent application No. 2002-101722 that the laser characteristics of modules having the semiconductor laser elements with an oscillation wavelength of 350 to 450 nm and being contained in a sealed container deteriorate when the oxygen concentration in the sealing atmosphere become too high.
Oxygen-concentration dependence of the aging deterioration rates of laser modules using semiconductor laser elements has been examined, and variations of the reliability of the laser modules with the oxygen concentration in the sealing atmosphere, where the cleaning process as disclosed in the document (5) is performed on the laser modules has been evaluated, and the above examination is performed for different oscillation wavelengths of the semiconductor laser elements, 410 nm, 810 nm, and 980 nm. According to the evaluation, the effect of improving the laser characteristics with increase in the oxygen concentration is not observed in the laser modules using semiconductor laser elements which have the wavelength of 410 nm, while such an effect is observed in the laser modules using semiconductor laser elements which have the infrared wavelengths of 810 nm and 980 nm.
That is, in the case where the semiconductor laser elements in the laser modules have an infrared wavelength of 810 or 980 nm, the decomposing reaction of hydrocarbon-based organic compounds which are deposited on surfaces of optical elements (including a light-entrance end face of an optical fiber and lenses) arranged on the path of laser light is enhanced with increase in the oxygen concentration, and therefore the reliability with the passage of time is increased with increase in the oxygen concentration.
On the other hand, in the case where the semiconductor laser elements in the laser modules have a wavelength of 410 nm, the reliability with the passage of time decreases when the oxygen concentration becomes 100 ppm or more. This is because when the oxygen concentration becomes 100 ppm or more, the amount of silicon compounds deposited on the light-entrance end face of the optical fiber at which the light beams are collected becomes significant. Since the silicon compounds absorb light as well as the hydrocarbon-based organic compounds, the deposited amount of the silicon compounds significantly decreases the aging reliability in continuous oscillation.
That is, the hydrocarbon deposit which is produced by the reaction of hydrocarbon gas with laser light is decomposed into carbon dioxide (CO2) and water (H2O) in an atmosphere containing at least a predetermined amount of oxygen, and is then removed. However, the deposited silicon compounds cannot be decomposed or removed by merely mixing oxygen into the atmosphere. The deposited silicon compounds are produced by a photochemical reaction of gas of an organic compound containing the silicon atom with laser light. For example, the organic compound contains a siloxane (Si—O—Si) bond or a silanol (—Si—OH) radical. When oxygen is contained in the atmosphere, the above photochemical reaction is accelerated. Hereinafter, the organic compound containing the silicon atom is referred to as the organic silicon compound.
The silicon compounds considered in this specification are organic and inorganic compounds having any structures which contain the silicon atom. The silicon compounds include inorganic silicon oxides (SiOx), organic silicon compounds, silicon-carbide compounds, and organic silicon-carbide compounds. In addition, silicon-based materials which are used at arbitrary places during manufacture of each laser module emit gas of organic silicon compounds. When a silicon-based material adheres to a surface of an element in the laser module, and the element is sealed in the laser module for use, a very small amount of organic silicon compound gas is emitted into the sealed atmosphere.
It is impossible to completely remove the gas component which exists during the manufacturing process, by merely providing equipment for purifying the sealing gas in a conventional clean room. In order to remove the gas component, an enormous amount of equipment investment is required. In addition, even when the process for oil removal from and cleaning of the inside of the laser module is performed as disclosed in aforementioned document (5), it is impossible to prevent mixture of organic silicon compound gas into an atmosphere during a manufacturing process.
As described above, even in the case where oxygen is contained in the sealing atmosphere in order to prevent accumulation of hydrocarbon compounds, when the oxygen content becomes too great, the deposited amount of silicon compounds increases, and the laser characteristics deteriorate. Further, since the end portion of the optical fiber and other optical elements including lenses are fixed to the inside of each laser module with an adhesive or brazing material, it is impossible to periodically replace the optical elements as disclosed in aforementioned document (2).
It is considered that the output power and light intensity of laser apparatuses will be further increased in the future as in the optically-multiplexing laser-light sources. In particular, in laser apparatus which output a high-energy laser light in the ultraviolet range, the power density increases at the light-emission end face of the optical fiber as well as the light-emission end faces of the plurality of semiconductor laser elements, the light-entrance end face of the optical fiber, and the other optical elements. Therefore, the organic compounds are likely to be decomposed, and contaminants such as decomposition products and dust in the atmosphere are likely to adhere to the above-mentioned end faces and optical elements.